This invention relates to the field of optical fibre telecommunications, and more particularly to telecommunications over long distances. The invention is particularly concerned with optical communication systems using return-to-zero (R-Z) pulses, and the problems associated with distortion in the pulse shapes over long distances.
It is Known that a xe2x80x9csolitonxe2x80x9d type signal can be propagated along a dispersive fibre without significant chromatic dispersion, because the dependence of the refractive index on the intensity of the signal is able to oppose chromatic dispersion. Thus, the spectral form of the signal is preserved over distance.
There are, however, numerous other distortions which arise during the transmission of optical pulses, especially when they are not exact solitons. These can lead to timing jitter or amplitude noise.
It has been recognised that it is desirable to constrain timing jitter and amplitude noise without the need for full signal regeneration so that the distance between regenerators can be increased. One known approach for improving timing control involves the use of a synchronous modulator which truncates the rise and fall edges of the optical pulses. Appropriate control of a modulator in this way can also reduce amplitude noise at the same time. For example, a pulse having increased amplitude as a result of amplitude noise will, after passage through the timing control modulator, have larger bandwidth than pulses of lower amplitude. Optical filtering gives rise to increased loss of higher bandwidth signals, thereby bringing down the amplitude. Thus, it has been appreciated that the use of timing control may combine with the non-linear characteristics of the optical fibre and optical filtering in such a way that the optical pulses tend towards a uniform amplitude. Optical filtering elements may be used to achieve the desired filter characteristics.
A problem with this approach is that it places constraints upon the fibre dispersion characteristics, the span lengths and the power budgets for the signals. All of these considerations need to be taken into account to enable a system to be designed which achieves the desired control of the pulse shape. This approach is not suitable for low power systems, such as WDM optical transmission systems.
According to the invention, there is provided an apparatus for reshaping optical pulses comprising an Nxc3x97N optical coupler having N ports on each side of the coupler, where N is equal to or greater than 3, wherein an input and an output of the apparatus are defined by two of the N ports on one side of the optical coupler, and a length of optical fibre connects two of the N ports on the other side of the optical coupler, and wherein the input and output ports are selected such that the function of transmission versus input power has a negative gradient at the power level of the optical pulses.
The invention essentially provides an optical loop mirror, using a coupler having more than two input and output ports. By virtue of phase shifts within the coupler, the transmission function can be selected such that higher power pulses are attenuated more than lower power pulses, which tends to even out pulse powers to a constant level. This helps maintain a constant signal to noise ratio within the system and can extend the number of optical spans of an optical signal before it requires full electrical regeneration.
The reshaping apparatus is effectively implementing a non-linear transfer function which attenuates optical pulses according to their power.
Preferably, the function of transmission versus input power decreases from a first transmission value at substantially zero input power to a minimum transmission value at a second input power, and the optical pulses have power in the range from zero to the second input power level.
The apparatus for reshaping optical pulses of the invention is a passive optical component which does not require accurate timing control in dependence upon the arrival of the optical pulses. The apparatus can therefore be implemented at low cost.
Preferably, a device is provided in the length of optical fibre to introduce asymmetry, for example an optical amplifier or optical attenuator.
To achieve the desired optical transmission function, the optical coupler may comprise a 3xc3x973 optical coupler, and the ports on the one side then comprise two outer ports and a middle port, and the ports on the other side comprise two outer ports and a middle port, and wherein the input port comprises one of the outer ports on the one side, the output port comprises the other of the outer ports on the one side, and the length of optical fibre connects the two outer ports on the other side of the optical coupler.
The invention also provides an optical regeneration unit for reshaping optical pulses, including an apparatus for reshaping optical pulses of the invention. The invention also provides a multi-channel optical transmission system comprising a transmitter and a receiver and at least one optical regeneration stage, the optical regeneration stage comprising an optical demultiplexer for receiving a multi channel signal and for dividing the signal into the individual channels, at least one optical regeneration unit of the invention.